The present invention relates to a digitizer constituted by a tablet and a position indicator and designed to detect a position indicated by a position indicator placed on the tablet.
In conventional digitizers, especially a cordless digitizer in which the tablet and the position indicator are not connected by wire lines, an oscillator, a first loop coil group, constituted by a plurality of loop coils insulated from each other, and a second loop coil group which is perpendicular to the first loop coil group, are arranged on the tablet side, and a coil is arranged on the position indicator side.
A position or the like indicated by the position indicator placed on the tablet is detected as follows. Frequencies corresponding to the respective resonance frequencies (to be described later) of the position indicator are selected. Drive currents of the selected frequencies are sequentially supplied from the oscillator to the first loop coil group to sequentially generate magnetic fields from the first loop coil group, thus causing electromagnetic coupling between the first loop coil group and the coil of the position indicator. The indicated position is then detected on the basis of the values of induced voltages generated in the second loop coil group by this electromagnetic coupling.
More specifically, the position indicator includes a plurality of capacitors and switches in addition to the above-mentioned coil. One of the capacitors is connected in parallel with the coil to constitute a resonance circuit, while a plurality of parallel pairs of the switches and the capacitors are connected in series with the coil of the resonance circuit. In the position indicator, therefore, when the respective switches are depressed, resonance circuits having different resonance frequencies are formed. On the tablet side, the respective resonance frequencies of the position indicator are detected to check which switch is depressed. That is, in the tablet, frequencies corresponding to the respective resonance frequencies are selected, and drive currents of the selected frequencies are sequentially supplied from the oscillator to the first loop coil group, thus detecting the values of induced voltages generated in the second loop coil group. Whether any one of the switches is depressed or which switch is depressed is checked on the basis of the frequency of a drive current supplied when a predetermined induced voltage value is detected. If a predetermined voltage value is not detected, a detecting operation of sequentially selecting frequencies corresponding to the respective resonance frequencies and supplying drive currents of the selected frequencies to the first loop coil group is repeatedly performed until the predetermined induced voltage value is detected.
When switch depression information associated with the position indicator is to be detected on the tablet side, a drive current of a reference frequency is applied from the tablet to the first loop coil group. The switch depression information can be detected on the basis of the difference between the phase of the drive voltage and that of an inducted voltage generated in the second loop coil group.
In addition, the above-described position indicator may be formed into a pen-like shape so that various input operations can be performed by using the position indicator like a pen.
In this case, the capacitor connected in parallel with the coil of the position indicator is of a variable capacitance type. This variable capacitor changes its capacitance in accordance with the pressing force (writing pressure) of the pen-like position indicator with respect to the tablet.
A digitizer of this type, therefore, detects not only an indicated position but also the writing pressure of the pen-like position indicator with respect to the tablet.
In some digitizers, the above-described oscillator is arranged on the position indicator side. In this arrangement, the coil of the position indicator is oscillated at a predetermined frequency by the oscillator, and magnetic fields generated by this oscillation of the coil are detected as induced voltages generated in the first and second loop coils, thereby detecting a position indicated by the position indicator on the basis of the values of these induced voltages.
As described above, the conventional digitizers use electromagnetic coupling between the tablet and the position indicator. Therefore, either in the scheme of generating magnetic fields from the tablet side or in the scheme of generating magnetic fields from the position indicator side, the apparatus includes an oscillator for always generating magnetic fields. Accordingly, the apparatus is complicated, and hence cannot have a compact, economic arrangement.
Furthermore, when depression information of any one of the switches of the position indicator is to be detected, the respective resonance frequencies of the position indicator are selected, and drive currents of the selected frequencies are supplied to the first loop coil group until a predetermined induced voltage corresponding to the switch depression information is obtained. Therefore, detection of this information requires a long period of time.
When switch depression information is detected on the basis of the difference between the phase of a drive voltage of a reference frequency which is applied to the first loop coil group and the phase of an induced voltage generated in the second loop coil group, only frequencies in a frequency range near the reference frequency can be used. If, therefore, a large number of switches are arranged in the position indicator, a depressed switch cannot be identified, resulting in a decrease in resolution in switch detection.
Moreover, in the apparatus which uses the variable capacitor for the coil of the position indicator to have a writing pressure detecting function as an additional function, high-speed, high-resolution detection of writing pressure information is difficult to achieve for the same reasons as those described with reference to the detection of switch depression information.